1. Field of the Invention
The present invention generally relates to a projection-type image display apparatus for use in systems such as a compact projection-type color liquid crystal television system and an information display system, and more particularly, the present invention relates to a projection-type image display apparatus for achieving color display using three image display devices.
2. Description of the Related Art
In a liquid crystal display device, a driving voltage is independently applied to pixel electrodes regularly arranged in a matrix. Optical characteristics of a liquid crystal material are changed in response to the voltage application, whereby images, characters, and the like are displayed. Methods for independently applying a driving voltage to such pixel electrodes as described above include a simple matrix address method, and an active matrix address method in which non-linear 2-terminal elements or 3-terminal elements are provided in the liquid crystal display device.
In the active matrix address method, provision of switching elements such as MIM (Metal-Insulator-Metal) elements and TFT (Thin Film Transistor) elements, and signal lines for supplying a driving voltage to the pixel electrodes is required. In the case where high-intensity light is incident on such switching elements, a resistance of the switching elements during an OFF state is reduced. Therefore, charges accumulated during application of a voltage are discharged. Moreover, a driving voltage is not properly applied to portions of such a liquid crystal which are in a region where the switching elements or the line electrodes are formed. As a result, a correct display operation is not performed, and leakage of light occurs even in a black state, causing reduction in contrast ratio.
Accordingly, in the case where a transmission-type liquid crystal display device is used, light must be prevented from entering the above-mentioned region. This is achieved by providing a light-shielding element, i.e., a black matrix 102, as shown in FIG. 10, on a substrate facing a TFT substrate with a liquid crystal layer interposed therebetween. The switching elements such as a TFT 101 and pixel electrodes are provided on the TFT substrate. As a result, light is prevented from entering the transmission-type liquid crystal display device by the TFT 101, a gate bus line 103, and a source bus line 104 each having a light-shielding property, as well as by the black matrix 102. Accordingly, the area of an effective pixel opening in the pixel region, that is, the numerical aperture, is reduced.
Reduction in size of the switching elements and the signal lines is limited due to the electric performance thereof and limitation of the manufacturing technology. However, attempts have been made to make liquid crystal display devices smaller. In order to produce a smaller liquid crystal display device, the pitch between the pixel electrodes should be reduced. However, the smaller the pitch is, the more the numerical aperture is reduced.
Now, a conventional projection-type image display apparatus using such a liquid crystal display device as mentioned above will be described.
A projection-type color image display apparatus using a liquid crystal display device(s) is categorized into a single-plate type (mode) using only one liquid crystal display device, and a three-plate type (mode) using three liquid crystal display devices respectively corresponding to light in wavelength ranges of three primary colors, i.e., corresponding to light in red (R), blue (B), and green (B) wavelength ranges (hereinafter, referred to as R, G and B color light rays, respectively). In the single-plate type, an image formed by a liquid crystal display device having a color filter of R, G and B arrangement is projected onto a screen. The R, G and B arrangement of the color filter is of a mosaic type, a stripe type, or the like. Such a single-plate type is disclosed in, for example, Japanese Laid-Open Publication No. 59-230383. The single-plate type uses only one liquid crystal display device, and therefore, an optical system required in the single-plate type is simple as compared to the case of the three-plate type. Accordingly, the single-plate type is suitable for reduction in cost and size of the projection-type system. However, about 2/3 of light emitted from a light source is absorbed by the color filter. Therefore, illuminance is generally low in the single-plate type.
On the other hand, in the three-plate type, an optical system for dividing white light emitted from a light source into R, G and B color light rays, and liquid crystal display devices for forming an image corresponding to the respective color light rays are separately provided so as to make the respective images optically overlap each other to produce full-color display on a screen. In this structure of the three-plate type, a mechanism for adjusting light convergence in the three liquid crystal display devices, and optical parts for separating/synthesizing R, G and B color light rays are required, making the structure more complex. However, white light emitted from a light source can be utilized effectively, whereby higher luminance can be achieved as compared to the single-plate type.
The numerical aperture is reduced as a liquid crystal display device is reduced in size, as described above. Accordingly, in the case where a smaller transmission-type liquid crystal display device is used in a projection-type image display apparatus, sufficient brightness is less likely to be obtained even by the three-plate type. Therefore, a projection-type image display apparatus using a reflection-type liquid crystal display device has been developed in order to solve this problem.
In a reflection-type liquid crystal display device, a reflective pixel electrode 100 can be formed on the TFT 101 serving as a switching element, as shown in FIG. 11. Therefore, in the case where the reflection-type liquid crystal display device and the transmission-type liquid crystal display device are the same in size, the reflection-type liquid crystal display device can achieve higher numerical aperture than that of the transmission-type liquid crystal display device. Such higher numerical aperture is very effective to improve brightness achieved by the projection-type image display apparatus.
A combination of a polarization beam splitter (hereinafter, referred to as PBS) for dividing white light emitted from a light source into p-polarized light and s-polarized light and a cross dichroic prism for dividing white light into R, G and B color light rays, together with such a reflection-type liquid crystal display device has been proposed in Japanese Laid-Open Publication No. 4-338721. However, Japanese Laid-Open Publication No. 4-338721 mentions neither the fact that spectral characteristics of the cross dichroic prism are different between p-polarized light and s-polarized light entering the cross dichroic prism, nor the influence of such a difference upon the display characteristics of projection. These problems will be discussed below.
In addition, Japanese Laid-Open Publication No. 4-319910 discloses a projection-type liquid crystal display apparatus using a light-scattering liquid crystal display device as a reflection-type liquid crystal display device and including a Schlieren optical system. However, Japanese Laid-Open Publication No. 4-319910 does not mention the influence of the use of the light-scattering liquid crystal display device upon the color-separation characteristics of the cross dichroic prism.
The above-mentioned Japanese Laid-Open Publication No. 4-338721 discloses an apparatus including a reflection-type liquid crystal display device in a birefringence mode. According to the publication, incident light to the cross-dichroic prism enters into the reflection-type LCD device and is reflected thereby. The reflected light enters into the cross-dichroic prism again. The polarization direction of the incident light is rotated by 90.degree. from that of the reflected light. The cross dichroic prism is generally highly dependent upon the polarization state at its reflection plane. Accordingly, the spectral characteristics of the cross dichroic prism in the case of dividing white light into R, G and B color light rays are significantly different from those in the case of synthesizing the divided color light rays.
FIG. 12A shows an example of the spectral characteristics of the cross dichroic prism with respect to reflected B color light rays having s-polarization and p-polarization. FIG. 12B shows an example of the spectral characteristics of the cross dichroic prism with respect to reflected R color light rays having s-polarization and p-polarization. As can be seen from FIGS. 12A and 12B, p-polarized light has a smaller wavelength range than that of s-polarized light in the case of R and B color light rays. In the case of a G color light ray, s-polarized light has a smaller wavelength range than that of p-polarized light. Consequently, the brightness and the range of color reproduction of projection highly depend upon the p-polarized light having a smaller wavelength range in the case of the R and B color light rays, while highly depending upon the s-polarized light having a smaller wavelength range in the case of the G color light ray, in spite of the fact that both s-polarized light and p-polarized light enter the cross dichroic prism. Accordingly, R and B color light rays are restricted by p-polarized light, whereas a G color light ray is restricted by s-polarized light.
In such a case, when the brightness and the range of color reproduction of the R and B color light rays are adjusted, the wavelength range of the G color light ray is reduced as shown by the shaded portion of FIG. 13A, adversely affecting display and white balance of the G color light ray. On the other hand, when the brightness and the range of color reproduction of the G color light ray are adjusted, respective wavelength ranges of the R and B color light rays are reduced as shown by the shaded portions of FIG. 13B, adversely affecting display and white balance of the R and B color light rays.
Moreover, in the case where the spectral characteristics are different between p-polarized light and s-polarized light, the following problems will occur. As shown in FIG. 14, when-s-polarized light, for example, enters a cross dichroic prism 14, s-polarized light (a) in the R wavelength range is reflected at the cross dichroic prism 14 into a reflection-type liquid crystal display device 5-R corresponding to an R color light ray. Then, the s-polarized light (a) is reflected by the reflection-type liquid crystal display device 5-R with its polarization state being changed in accordance with an image signal. The resultant light contains a p-polarized light component. Therefore, light (b) corresponding to the portion a in FIG. 12B which will be described later passes through an R reflection plane of the cross dichroic prism 14 into a reflection-type liquid crystal display device 5-B corresponding to a B color light ray. This p-polarized light is again changed to s-polarized light by the liquid crystal display device 5-B. In this case, the resultant s-polarized light (c) will not be reflected back to the liquid crystal display device 5-R, but to a reflection-type liquid crystal display device 5-G corresponding to a G color light ray. The s-polarized light is again changed to p-polarized light by the liquid crystal display device 5-G. In this case, the resultant p-polarized light (d) is reflected from the liquid crystal display 5-G, passing through the R reflection plane of the cross dichroic prism 14 into a projection lens. As a result, the light is projected onto a screen as stray light or a ghost image.
The same problems of stray light and ghost images as described above will also occur in the case of a B color light ray. Even in the case where p-polarized light is directed into the cross dichroic prism 14, the problems of stray light and ghost images will occur based on the same principles.
In Japanese Laid-Open Publication No. 4-319910, a light-scattering liquid crystal display device is used. Therefore, basically, random polarized light enters a cross dichroic prism. Accordingly, such shifting of a spectrum between incident light and outgoing light as generated in the case of the reflection-type liquid crystal display device in a birefringence mode will not occur. However, with respect to the random polarized light, the cross dichroic prism will have mean spectral characteristics of p-polarized light and s-polarized light at its color separating plane. As a result, the spectral characteristics have a less sharp rise or fall at the boundary between transmission and reflection, showing a stepped shape at the transmittance of about 50%, as shown in FIGS. 15A and 15B. Therefore, color purity of R, G and B wavelength ranges as well as light utilization efficiency are significantly degraded.